This application is related to two U.S. applications having Ser. No. 505,978 filed June 20, 1983 and now U.S. Pat. No. 4,561,069 and Ser. No. 513,610 filed July 14, 1983, and now U.S. Pat. No. 4,486,858 which are assigned to the Assignee of the present application.
1. Field of the Invention
The present invention relates to a magnetic-bubble memory device (hereinafter referred to as a bubble memory) and more particularly to a bubble propagation track defined by elements or patterns of magnetically soft material.
2. Description of the Prior Art
There are various known types of bubble memories, the most common one being an in-plane field access type of bubble memory which includes a thin layer of magnetic material in which magnetic bubbles can be propagated along propagation tracks in response to a magnetic drive field rotating or reorienting cyclically in the plane of the magnetic material layer. An in-plane field access type of bubble memory in which the bit period is 8 micrometers (.mu.m) and the capacity is 1 megabit (Mb) has been realized, and a 4 .mu.m period, 4 Mb bubble memory is now being developed.
There are two well-known types of propagation tracks, one being defined by elements or patterns of magnetically soft material such as permalloy and commonly called a "permalloy propagation track" and the other being defined by an ion-implanted pattern and commonly called an "ion-implanted propagation track". A 4 .mu.m period ion-implanted track can be easily fabricated and is a very effective means for realizing a 4 .mu.m period, 4 Mb bubble memory. However, superior-performance function gates for a 4 Mb bubble memory having a 4 .mu.m period ion-implanted track, particularly block-replicate gates for major-minor loop-organized bubble memories, are still in the process of development.
Superior-performance function gates for a bubble memory with a permalloy track have already been realized. However, in realizing a 4 .mu.m period permalloy track, there is a gap problem. There are known gap-tolerant permalloy propagation patterns, such as half-disk and asymmetric chevron patterns. However, an allowable gap for a gap-tolerant pattern is, at the most, one eighth of the period. Therefore, in a 4 .mu.m period permalloy track defined by gap-tolerant patterns, the gaps should have a width of 0.5 .mu.m or less, which cannot be achieved by present-day photolithographic resolution.
New permalloy propagation patterns, called wide-gap patterns, have been reported by A. H. Bobeck et al (EA-1, 3M conference, Atlanta, 1981). In a wide-gap track defined by wide-gap patterns (explained in detail with reference to the accompanying drawings), superior bubble propagation performance can be obtained with gaps one fourth of the period. Therefore, a wide-gap pattern is a very promising means for realizing a 4 .mu.m period using a 1 .mu.m gap permalloy bubble propagation track which can be fabricated by present-day photolithography, thus realizing a 4 .mu.m period, 4 Mb bubble memory.
In the design of bubble propagation tracks, 90.degree. and 180.degree. turns are important. In particular, for a folded a doubled-back minor loop in a major-minor loop-organized bubble memory, 180.degree. turns are indispensable. Several turn designs for the wide-gap permalloy propagation track mentioned above have been reported by A. H. Bobeck et al. However, as explained below, poor propagation performance is observed for the reported wide-gap track turns.